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FDP Forum / Amp Mods, Repairs, and Projects / Montreux Troubleshooting; Noise in Channel 2

70Mach1

United States

Oct 5th, 2018 12:54 PM   Edit   Profile  

Hi,

New member here.

I'm working on a Fender Montreux for a friend. The issue is static-y noise in Channel 2. Channel 1 sounds fine. It doesn't seem to be oxidation in the volume pots, as it's unaffected by turning them.

Using a 1kHz sine wave and a signal tracer, the issue seems to originate in the amp board. The sine wave is clear at the input and accompanied by the static at the output. It SEEMED that the problem was localized at C7 and R18, since the noise was present on one side of these two, but not the other. I pulled both, along with R19, but they all measure ok. I did a basic go no-go short test on the output transistors, Q8 and Q9, and both checked out. In fact the only components that are out of spec are R12 and R13, both of which have drifted a little above tolerance. I still need to check the transistors on the board.

Any suggestions/guidance from those who've worked on this amp (or the Showman or London Reverb) would be appreciated.

DrGonz78

United States Phx,AZ

Oct 5th, 2018 09:43 PM   Edit   Profile  

A big point of issue to consider with those amps are ribbon cables that connect between the boards. Not really thinking this sounds likely to be the problem but I would still check them out. I would chopstick around (delicately) wiggling those connection points to make sure they are not the issue. Possibly just ohm out each connection on the ribbon cables to make sure they are making good contact. There are ground points through those ribbon cables and I would make sure they are not the issue here.

For everyone to follow the schematic it can be found here
http://www.stratopastor.org.uk/strato/amps/twoseriessolidstatefenders/montreux.pdf

(This message was last edited by DrGonz78 at 11:49 PM, Oct 5th, 2018)

DrGonz78

United States Phx,AZ

Oct 5th, 2018 09:59 PM   Edit   Profile  

One more point... if the problem was in the power amp portion of the amp then I would think that the noise issue would not be isolated to channel 2 only. Channel 1 outputs right into that same part of circuit with out the issue. My thought is that channel 2 noise has to be originating just before power amp circuit. Or grounding issue in that channel that spurs the noise.

70Mach1

United States

Oct 6th, 2018 06:36 AM   Edit   Profile  

Thanks DrGonz78.

Good point about the ribbon cables. I'll check them as you suggest.

I have to study the amp board schematic more closely, but everything there is shared with both channels?

DrGonz78

United States Phx,AZ

Oct 7th, 2018 12:43 AM   Edit   Profile  

Well maybe your referring to C7 on the preamp board? Your original post just says originated in the "amp board." I took that to mean C7 on power amp page of the schematic. R18 & R19 are very much in that same area too. So which board are we referencing?

70Mach1

United States

Oct 8th, 2018 01:16 PM   Edit   Profile  

I was referring to the amp board, not the preamp board. But you're right that it isn't originating there. Not sure what I was I thinking...

After some further signal tracing, I found that it is, in fact, coming from the preamp board. At this point I've narrowed it down to C35 and where it connects to IC5. Haven't gone any further yet.

70Mach1

United States

Oct 8th, 2018 05:08 PM   Edit   Profile  

Ok,update to the update. The noise was actually originating from R44 on the preamp board, which is labeled "BIAS" on the schematic and "OFFSET" on the board. Exercising this trimmer a bit (and then returning it to its original position) seems to have cleared up the issue.

DrGonz78

United States Phx,AZ

Oct 9th, 2018 02:00 AM   Edit   Profile  

Had similar issues with an old digitech delay, PDS1002. Thing has like 5 biasing pots and they are the kind that attract dirt. Found a few dried up electrolytic caps and the thing still would not work. Started in on the pots and to my surprise turning them cleaned up the the tracks contacts. The unit just springed back to life. Glad to hear that you figured out the issue with the amp, nice job!

(This message was last edited by DrGonz78 at 04:01 AM, Oct 9th, 2018)

70Mach1

United States

Oct 9th, 2018 12:06 PM   Edit   Profile  

Thanks. It's definitely much better, though there's still a bit more background hiss through Channel 2 with the volume cranked than I'd like.

Any idea what the bias settings should be for the preamp board and amp board? I'm used to dealing with home audio amps where there's a service manual with this info. As is, I just have the owner's manual and schematics.

Stratopastor

Stockton-on-Tees

now even awesomer
Oct 9th, 2018 03:10 PM   Edit   Profile  

Hi - just saying, I read this with interest (in case my London goes noisy) and I'm glad you got it sorted. I collect serial numbers for all the Rivera-era amps (yes, I should get out more) and if you let me know the serial number of your Showman I'll give you a rough date for it. My email address is on the stratopastor pages.

DrGonz78

United States Phx,AZ

Oct 10th, 2018 04:59 AM   Edit   Profile  

Yes do send Stratopastor the serial numbers to add to his online collection. His site I attached to this thread since he so graciously documented so much info about these amps. Thanks again Stratopastor for the made in Japan version of the Stage ii Fender schematic that you emailed to me some years ago.

As far hiss is concerned. Perhaps some of those old opamps are the culprits? Perhaps ic3 or ic5 could be substituted for lower noise options. That R44 bias pot is a bit strange and not really sure what it does, so I would typically leave it set the way it was. You don't really need to bias the output of the amp either. Kinda the old axiom if it ain't broken don't fix it.

70Mach1

United States

Oct 10th, 2018 04:50 PM   Edit   Profile  

I'll have to get the ok from my friend who owns the amp, but I doubt he'll have any objection to me sending the serial number to you Stratopastor.

DrGonz78, I'll leave the bias pots alone as you suggest. One other issue I found with this amp was bad/broken solder joints where the Master Volume connects. Took care of that and also reflowed some suspect joints at a couple of the ribbon cable connections.


Stratopastor

Stockton-on-Tees

now even awesomer
Oct 11th, 2018 01:19 AM   Edit   Profile  

On my London I've had trouble, mostly self-inflicted, with the ribbons and the posts they connect with! anyone else opening up a London, Montreux or s/s Showman, be ready to re-check things every time you disturb a ribbon cable.... :-)

Gonz, thanks for the kind words...

FDP Forum / Amp Mods, Repairs, and Projects / Montreux Troubleshooting; Noise in Channel 2




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